1. Field of the Invention
The present invention relates to a slot array antenna and more particularly to the arrangement of a feed port formed in a slot array antenna.
2. Description of the Background Art
Generally, an array antenna has a plurality of antenna elements or segments arranged in a certain pattern for acquiring characteristics impractical with a single antenna. Further, by regulating the respective antenna elements in phase, it is possible to control the directivity of the entire array antenna.
Today, frequency bands allocated to a variety of communication apparatuses are becoming short due to the remarkable development of radio transmission technologies. To make up for short frequency bands, it is necessary to more effectively use frequencies and to further shift frequencies to a higher range. Technologies meeting such requisites are therefore an urgent issue. Recently, a millimeter wave, for example, that has not been taken into consideration except for fundamental studies is planned for the application to ITS (Intelligent Transport System). In motorized societies, millimeter-wave transmission apparatuses are expected to be used as popularly as household appliances in the near future.
Under the above circumstances, the application of various parts and devices to the millimeter-wave range is, of course, necessary in the millimeter-wave transmission field. An antenna is one of the most important devices for millimeter-wave transmission systems. Millimeter transmission systems are not practicable without an antenna capable of transmitting and receiving millimeter-wave signals. Today, research institutions and manufacturers joining in the worldwide study and development of millimeter-wave transmission systems are competing with each other for high performance millimeter-wave antennas. While some different millimeter-wave antennas have already been proposed, one of them featuring high performance is a slot array antenna.
The slot array antenna is made up of a plurality of conventional slot antennas or antenna elements arranged in a certain pattern. The slot antennas are sized and arranged in a particular pattern implementing a desired electric field distribution in a certain region. For example, the type of slot antennas arranged bidimensionally in a rectangular region can have an electric field which distributes uniformly in direction, phase and amplitude. Theoretically, the slot array antenna is substantially the same in radiation characteristics as the aperture antenna having a uniform electric field distribution. However, the slot array antenna is superior to the aperture antenna when it comes to the freedom of configuration and the uniformity of the electric field distribution.
FIG. 1 shows the basic configuration of a conventional, bidimensional array antenna. As shown, the array antenna includes feed port or signal generator 20 and antenna elements or segments 21. Transfer paths 22 connect the feed port 20 with antenna elements 21. At the same time, the transfer paths 22 play the role of phase shifters. More specifically, each of the transfer paths 22 determines the phase of an electromagnetic wave to be radiated from one of the antenna elements 21 associated therewith, and has critical influence on the radiation characteristics of the entire array antenna. To further adjust the phases, additional phase shifters may serially be arranged on respective transfer paths, as the case may be.
FIG. 2 shows a specific configuration of a slot array antenna using a single, rectangular waveguide tube. As shown, the slot array antenna includes a waveguide 31 formed with slots 32 in one of the walls thereof. Usually, each slot 32 has its length that is equal to about one-half of the wavelength xcex of an electromagnetic wave input to the waveguide 31, and its width that is equal to about one-twentieth of the wavelength xcex. In the specific configuration, when the waveguide 31 is driven in the dominant mode TE10, the magnetic and electric fields are distributed in the directions of the length and the width of the slots 32, respectively.
The electromagnetic wave mode referred to in the present specification is the dominant mode TE10 unless otherwise stated explicitly. Generally, as shown in FIG. 2, the pitch between the slots 32 spaced in the longitudinal direction of the waveguide 31 is equal to about one-half of the guide wavelength xcexg. The pitch between the nearby slots 32 aligned in each of the longitudinal lines, or slot arrays, is substantially equal to the guide wavelength xcexg.
A desired distribution of electromagnetic fields can be set up to some extent on the outer wall 33 of the waveguide if the dimensions and position of the slots 32 cut in the wall 33 are appropriately adjusted. Such a slot array antenna is monodimensional. By arranging a plurality of slot array antennas in parallel, each of which has the configuration shown in FIG. 2, there can be implemented a bidimensional slot array antenna. Today, a bidimensional slot array antenna is recognized as one of high-gain antennas theoretically and experimentally, as discussed in a Japanese document, xe2x80x9cFundamentals and Applications of Millimeter-Wave Technologiesxe2x80x9d, REALIZE INC., Tokyo, Japan, pp. 140-184, Jul. 31, 1998.
FIG. 3 shows a conventional, bidimensional slot array antenna in an exploded view. A bidimensional, slot array antenna will be simply referred to as a slot array antenna hereinafter unless stated otherwise. As shown, the slot array antenna is generally made up of a slot plate 411 and a waveguide plate 412 forming waveguides. Generally, the slot plate 411 is implemented by a thin, electro-conductive plate and formed with a plurality of slots 421, see FIG. 4. The plate 412 is a relatively thicker, electro-conductive plate having rectangular-cross-sectional grooves formed therein. The grooves are configured such that an input electromagnetic wave can be fed from a single feed port to all of the slots 421 of the slot plate 411. More specifically, when the slot plate 411 is laid on and adhered to the plate 412 into an assembly, the arrays of slots are positioned right above the grooves associated therewith with parts of the plate 411 forming the walls of waveguides established by those grooves. In this arrangement, the entire assembly operates as a slot array antenna. The higher the conductivity of the conductors constituting the slot plate 411 and the plate 412, the smaller the ohmic loss of the entire antenna. Further, the accuracy in assembly and adhesion of the slot plate 411 with the plate 412 noticeably influences on the radiation characteristics of the resultant antenna.
As shown in FIG. 4, while each slot 421 formed in the slot plate 411 is basically rectangular, its opposite ends are sometimes rounded for manufacturing reasons. Each slot 421 has its length that is equal to about one-half of the wavelength of an electromagnetic wave to be radiated, and its width that is equal to about one-twentieth of the same, as stated earlier. The pitch between the nearby slots 421 on the same array is substantially equal to the guide wavelength xcexg.
As seen in FIG. 5, the plate 412 has a feed port 431 cut therein. When the slot plate 411 and plate 412 are adhered together into assembly, a portion 432 indicated by a dashed ellipse in FIG. 5 constitutes an H plane tee junction as referred to in the art of microwave circuit devices. An electromagnetic wave input via the feed port 431 is split into two at the H plane tee junction 432 in the opposite directions perpendicular to each other. The resulting two electromagnetic waves are of the same phase as to power.
A matching lug or post 433 plays the role of a matching stub included in a conventional H plane tee junction. The lug or post 433 protrudes toward the feed port 431, as seen in FIG. 5. A groove extending in the opposite directions from the H plane tee junction 432 constitutes a waveguide when the slot plate 411 and plate 412 are adhered together. Let this waveguide be referred to as a feed waveguide. Because the feed waveguide is symmetrical in the opposite directions with respect to the longitudinal axis of the feed port 431, the following description will concentrate only on one side of the plate 412 for the sake of simplicity.
Second feed ports 434 communicate the feed waveguide to a plurality of radiating waveguides 437. Each of the feed ports 434 has a sectional area substantially equal to the sectional area of the feed waveguide. Lugs or posts 435 protrude from the portions of the wall of the feed waveguide that face the feed ports 434, serving as matching stubs. The distance between the beginning of the feed waveguide to the last feed port 434 is selected to be equal to about one-fourth of the guide wavelength xcexg in order to suppress reflections.
The radiating waveguides 437 extend from the feed waveguide in the direction perpendicular to the latter via the second feed ports 434. Radiating waveguides 437 adjacent to each other are isolated from each other by a wall 436. The wall 436 splits the electromagnetic wave input via one feed port 434 adjoining it into two, so that the resulting two waves each are input to one of the two radiating waveguides 437. When the slot plate 411 and plate 412 are adhered together into assembly, each of the slot arrays positions above associated one of the radiating waveguides 437, thus functioning as the monodimensional array antenna shown in FIG. 2.
In the structure described above, the number of the radiating waveguides 437 constituting the conventional slot array antenna is a multiple of xe2x80x9c4xe2x80x9d without exception. If desired radiation characteristics and frequency to be used are determined, then the approximate number of radiating waveguides and that of slots to be positioned above each radiating waveguide are determined, determining the approximate size of the entire antenna.
FIG. 6 shows the slot array antenna, when assembled, having the slot plate and the waveguide plate adhered together. Because the electromagnetic wave mode inside the radiating waveguides is the dominant mode TE10, the magnetic and electric fields respectively extend in the lengthwise and widthwise directions of the slots. In FIG. 6, arrows 51 and 52 respectively indicate the directions of the magnetic and electric fields of the entire antenna. All the slots are oriented in the same direction, so that the electric fields around the primary surface of the antenna, except for the edges of the antenna, extend substantially in the direction 52. In this sense, the direction 52 may generally be representative of the direction of the polarized waves for the antenna.
While the characteristics of an antenna are generally required higher in gain and lower in side lobe level, the width and the polarization direction of a main beam and so forth are sometimes strictly restricted as well, depending on an application. For example, an anti-collision radar system expected to be mounted on a motor vehicle in the near future needs a couple of slot array antennas each having the above-described structure, one for transmitting and the other for receiving. The slot array antennas work in the linear polarization mode to both transmit and receive waves polarized in the same direction. Therefore on a motor vehicle, the transmitter and receiver antennas should only be mounted in the same position.
However, for example, assume that a first motor vehicle with a type of transmitter and receiver antennas meets on a road a second motor vehicle with the same type of transmitter and receiver antennas, but running in the opposite directions to each other. Then, when the first motor vehicle receives radio waves, it cannot separate the radio wave transmitted by itself and reflected by the second motor vehicle from another radio wave generated by the second motor vehicle. This problem occurs when the radio waves emanating from both anti-collision radar systems are polarized vertically and/or horizontally.
By contrast, if the slot array antennas are mounted on the first and second motor vehicles so that the polarization plane of the radio waves, and hence the body of the slot array antennas, is inclined by 45xc2x0 relative to the vertical or horizontal direction, then the polarization planes of the radio waves radiated from both motor vehicles are perpendicular to each other. The receiver antennas may therefore be mounted in the same manner as the transmitter antennas on the first and second motor vehicles, thus preventing each of the vehicles from receiving the radio waves radiated from the other. It follows that both of the transmitter and receiver antennas of the anti-collision radar system must be inclined by 45xc2x0.
In the conventional configuration, however, the first feed port 431 opens in one of the end faces of the plate 412, as shown in FIG. 5. As a result, when the on-board antenna is positioned such that the polarization direction is inclined by 45xc2x0 relative to the vertical or horizontal direction, the feed port, which is present in the above position and therefore must be inclined as well, obstructs the miniaturization of the anti-collision radar system. More specifically, the inclination of the feed port requires even its peripheral circuits including a feed circuit connected to the feed port to be rotated or skewed, thus requiring an additional space for accommodating the peripheral circuits, which must be skewed correspondingly.
It is an object of the present invention to provide a slot array antenna in which the polarization direction may freely be varied while requiring a minimum of additional space.
In accordance with the present invention, a slot array antenna has a feed port formed substantially at the center of the rear surface of the body of the antenna. More specifically, a slot array antenna having a plate-like structure includes a first feed port via which an electromagnetic wave is input, a feed waveguide for distributing the electromagnetic wave input via the first feed port, an array of second feed ports to which a particular electromagnetic wave distributed by the feed waveguide is input, and arrays of radiating waveguides to each of which the particular electromagnetic wave is fed. The first feed port is positioned substantially at the center of the length of the feed waveguide, preferably substantially at the center of one of opposite major surfaces of the plate-like structure.
Also, in accordance with the present invention, a slot array antenna includes a feed waveguide plate of electro-conductive material formed with a first feed port to which an electromagnetic wave is input, and a feed waveguide for distributing the electromagnetic wave input via the first feed port. A radiating waveguide plate of electro-conductive material is formed with an array of second feed ports to each of which a particular electromagnetic wave distributed by the feed waveguide is input, and arrays of radiating waveguides each being communicated to one of the second feed ports for receiving the particular electromagnetic wave. A slot plate of electro-conductive material is formed with arrays of slots for radiating the electromagnetic waves input via the radiating waveguides. The first feed port is positioned substantially at the center of the length of the feed waveguide. The feed waveguide plate has its front surface connected to the rear surface of the radiating waveguide plate while the radiating waveguide plate has its front surface connected to the rear surface of the slot plate.